Composite frame for x-ray tubes

ABSTRACT

An x-ray tube assembly ( 10 ) includes a frame ( 16 ) which defines an evacuated chamber ( 14 ). A central portion ( 40 ) of the frame which houses an anode ( 12 ) is formed from a thermally conductive liner ( 64 ) and a structural framework ( 62 ). The liner conducts heat away from the evacuated chamber to a surrounding cooling fluid. The framework provides windows ( 80, 80′, 80″, 82, 82′, 124 ), through which the liner is in direct thermal contact with both the cooling fluid and the evacuated chamber.

The present application relates to the x-ray tube arts. The inventionfinds particular application in connection with a composite frame for anx-ray tube which facilitates heat removal while retaining high strengthand rigidity and will be described with particular reference thereto. Itwill be appreciated, however, that the invention finds application in avariety of applications where it is desirable to transfer heatefficiently.

X-ray tubes include an evacuated envelope or frame which houses acathode assembly and an anode assembly. A high potential, on the orderof 100-200 kV, is applied between the cathode assembly and the anodeassembly. Electrons emitted by the cathode assembly strike a targetregion of the anode with sufficient energy that x-rays are generated.However, not all the energy is converted to x-rays. Rather, asubstantial portion of the energy is converted to heat, resulting inlocalized heating of the target and subsequently the envelope. In orderto distribute the thermal loading created during the production ofx-rays, a constant flow of a cooling liquid, such as a dielectric oil,is maintained around the frame throughout x-ray generation.

Conventionally, x-ray tube envelopes were formed of glass. Glass is easyto shape, inexpensive, and transmits thermal radiation. However, it hasseveral drawbacks. It is subject to cracking due to surface defects.Because glass is a brittle material, these failures are often rapid andunpredictable. Cracking also tends to occur when the glass is subjectedto a thermal gradient that is exacerbated if the glass is too thick.Glass is also subject to high voltage puncture and loss of insulatingproperties due to evaporated metal collecting on the surface.Particularly in computed tomography (CT) scanners, the increased gantryspeeds generate forces on the frame which glass envelopes are unable towithstand.

Metals such as copper, stainless steels, and nickel iron alloys, beganto replace glass as the material of choice for forming frames for highperformance applications, such as high speed CT scanners, while usingglass or ceramic for the cathode and anode end portions to provideelectrical insulation. These particular metals are of high purity toprovide low outgassing characteristics suited to vacuum environments.They are also able to withstand the high temperatures (about 500° C.)found in x-ray tubes. While copper is an effective thermal conductor, itis a relatively soft metal, due to the low yield point of annealedcopper. It has a tendency to creep (deform plastically) under hightemperatures and loads. Copper frames thus tend to distort under theforces generated at high rotation speeds, such as those in which thex-ray tube is rotated around a patient examination region in about asecond, or less. The distortion can lead to inaccuracies in maintainingthe position of the focal spot on the anode target. The tendency ofcopper to creep also affects baking out, the procedure used to processand clean out the tube, by limiting the bake out temperature of theframe.

With gantry speeds rising to about 120 rpm and demands for speed risingstill further for improved cardiac and other imaging, manufacturers havemoved to stainless steel for forming the frame. Although mechanicallystrong, stainless steel frames are not as efficient at transferring heatfrom one part of the frame to another as are copper frames.Additionally, transfer of the heat to the cooling liquid is slower thanfor copper. Localized heating of the frame tends to occur due to lowerrates of conduction of heat through the frame. As heat from the anodestrikes the stainless steel frame, the temperature of the frame can getsufficiently high that cooling oil breaks down. This is particularly aproblem around the x-ray tube window due to heating from the focal spotand secondary electrons. Carbon formed as a result of cooling oilbreakdown contaminates the oil, which can lead to arcing. The poweroutput of the x-ray tube is therefore limited by the capacity of theframe to transfer heat away from the x-ray tube.

The present invention provides a new and improved method and apparatuswhich overcome the above-referenced problems and others.

In accordance with one aspect of the present invention, an x-ray tube isprovided. The x-ray tube includes a frame which encloses an evacuatedchamber. An anode is disposed within the evacuated chamber. The frameincludes a vessel which surrounds the anode. The vessel includes a linerformed from a thermally conductive material which at least partiallydefines the evacuated chamber. A framework supports the liner and isformed from a structural material. The framework defines at least onethermal window therein through which the liner is in thermal contactwith both the evacuated chamber and a surrounding cooling fluid.

In accordance with another aspect of the invention, a method oftransferring heat from an x-ray tube to a surrounding cooling fluid isprovided. The method includes conducting heat from an evacuated chamberthrough a liner of the x-ray tube formed from a thermally conductivematerial. The liner is restrained against deformation with a structuralframework.

In accordance with another aspect of the invention, an x-ray tube isprovided. The x-ray tube includes a thermally conductive liner whichspaces an evacuated chamber of the x-ray tube from a surrounding coolingfluid. A structural framework reinforces the liner. The liner and theframework are stacked one within the other to form a vessel which housesan anode.

One advantage of at least one embodiment of the present invention is theprovision of an x-ray tube frame capable of withstanding the forcesgenerated at high gantry speeds.

Another advantage of at least one embodiment of the present invention isthat the frame is readily joined to other components of the x-ray tube.

Another advantage of at least one embodiment of the present invention isthat it enables efficient cooling of an x-ray tube and avoids localizedbreakdown of cooling oil.

Another advantage of at least one embodiment of the present invention isthat it enables the frame to be machined after brazing without providingspecial tooling to support the inside of the frame.

Another advantage of at least one embodiment of the present invention isthat it enables the focal spot and anode to cathode spacing to remainstable under large external forces that occur during scanning.

Another advantage of at least one embodiment of the present inventionresides in extended x-ray tube life.

Still further advantages of the present invention will become apparentto those of ordinary skill in the art. upon reading and understandingthe following detailed description of the preferred embodiments.

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating a preferred embodiment and are notto be construed as limiting the invention.

FIG. 1 is a perspective view of an x-ray tube assembly according to thepresent invention;

FIG. 2 is a side sectional view of a first embodiment of the x-ray tubevessel of FIG. 1;

FIG. 3 is a perspective view of the vessel of FIG. 2;

FIG. 4 is an exploded perspective view of the vessel of FIG. 2;

FIG. 5 is a is a side sectional view of a second embodiment of the x-raytube vessel of FIG. 1;

FIG. 6 is a perspective view of the vessel of FIG. 5;

FIG. 7 is a side sectional view of a third embodiment of the x-ray tubevessel of FIG. 1;

FIG. 8 is a perspective view of the vessel of FIG. 7;

FIG. 9 is a perspective view of a fourth embodiment of the x-ray tubevessel of FIG. 1; and

FIG. 10 is a side sectional view of the vessel of FIG. 9.

With reference to FIG. 1, an x-ray tube assembly 10 of the type used inmedical diagnostic systems, such as computed tomography (CT) scanners,for providing a beam of x-ray radiation is shown. The x-ray tubeassembly 10 includes an x-ray tube 11 comprising an anode 12, which isrotatably mounted in an evacuated chamber 14. The chamber is defined byan envelope or frame 16, shown partially cut away in FIG. 1. The x-raytube anode 12 is supported on a shaft 17 which is mounted for rotationabout an axis X via a bearing assembly shown generally at 18. A heatedelement cathode 20 supplies and focuses electrons A. The cathode isbiased, relative to the anode 12, such that the electrons areaccelerated to the anode. A portion of the electrons striking a targetarea of the anode is converted to x-rays B, which are emitted from thex-ray tube through an x-ray permeable window 22 in the frame.

The x-ray tube assembly 10 also includes a housing 30, filled with aheat transfer and electrically insulating coolant 13, such as adielectric oil. The housing 30 surrounds the frame 16 of the x-ray tube11. The cooling liquid is directed to flow past the window 22, the frame16, bearing assembly 18, and other heat-dissipating components of thex-ray tube assembly 10.

The frame 16 includes a bucket-shaped vessel 40 which defines the widestportion of the frame and surrounds the anode 12. The vessel 40 is indirect contact with the cooling oil 13. An upper end 42 of the vessel 40is closed by an annular cathode plate 44. The cathode plate 44 has acentral aperture 46 through which the cathode 20 extends. A housing orinsulator 48 for the cathode is welded or otherwise attached to thecathode plate 44 around the aperture 46. The terms “upper” and “lower”and the like are used with reference to the orientation of the x-raytube assembly illustrated in FIG. 1. It will be appreciated that theassembly, in operation, may have other orientations.

With reference also to FIG. 2, the vessel 40 diminishes in internaldiameter toward a lower end 50 thereof. In the illustrated embodiment,the vessel includes a side wall 52 including a cylindrical upper portion53, which is connected at its lower end with an annular base portion 54.The base portion 54 defines a central aperture 56 through which theanode shaft 17 extends. Around the aperture 56 is an annular weld flange57. The vessel 40 is mounted by the weld flange 57 to a lower portion 58of the frame which houses the bearing assembly. The lower portion 58 ofthe frame may be wholly or partially formed from glass or ceramic withmetal flanges to electrically isolate the anode from the cathode.

With reference also to FIGS. 3 and 4, the vessel 40 is a composite of athermally conductive material and a structural material. The thermallyconductive material provides a plurality of thermally conductivepathways 60 through the vessel for transfer of heat from the anode 12 tothe cooling liquid 13, while the structural material provides astructural framework or skeleton 62 which provides sufficient rigidityto the vessel to withstand the deformational forces caused by highgantry rotation speeds while providing thermal windows or cutouts forthe cooling liquid to make thermal contact with the evacuated chamber,via the thermally conductive passages. The thermally conductive passages60 are defined by a liner 64, supported by the framework 62.

The thermally conductive material is preferably one which has a thermalconductivity of at least 100 Watts/meter*degrees Kelvin, preferably, atleast 200 W/m*K, and most preferably, at least 350 W/m*K. The thermallyconductive material is preferably free or substantially free ofmaterials which have a tendency to outgas in the low vacuum conditionsof the x-ray tube. Suitable thermally conductive materials of this typeinclude copper, copper-beryllium alloys, other copper alloys, and thelike. For example, the thermally conductive material may be formed fromcopper, with copper being the primary element present. The thermallyconductive material preferably comprises at least 90% copper, morepreferably, at least 99% copper. At high purity, copper has a thermalconductivity of about 400 W/m*K. The thermal conductivity ofcopper-based materials tends to diminish as the proportion of alloyingmaterial or impurities increases. In contrast, stainless steels have athermal conductivity of 10-25 W/m*K. In general, the thermalconductivity of the structural material is less than that of thethermally conductive material, generally, less than half the thermalconductivity of the thermally conductive material.

The structural material is preferably one which has a yield strength ofat least about 1400 Kg/cm², more preferably, at least 2100 Kg/cm², asmeasured by ASTM D 882 or a similar test method. Exemplary structuralmaterials include ferrous materials, particularly stainless steel. Otherhigh strength materials suited for forming the framework includeInconel™ and other nickel alloys, titanium, Kovar™, and the like.Stainless steel has a yield strength of about 2800 to 3500 Kg/cm². Purecopper by comparison, has a yield strength of less than 700 Kg/cm². Ingeneral, the thermally conductive material may have a yield strengthwhich is less than that of the structural material, generally less thanhalf that of the structural material. The creep strength of thestructural material is preferably high. Preferably, the structuralmaterial has a minimum creep strength of 350 Kg/cm², more preferably 700Kg/cm² which is equivalent to 1% creep in 10,000 hours of service at500° C.

In the embodiment of FIGS. 2 and 3, the vessel 40 includes an innerliner 64 formed of the thermally conductive material, which is carriedwithin and contacts the framework 62. The liner 64 includes a side wall66, which includes a generally cylindrical portion 67, connected at itslower end with an annular base portion 68. The base portion defines acentral aperture 70 therein. As shown in FIG. 4, the window 22 of thex-ray tube 11 is set into a suitably shaped opening 72 in thecylindrical portion 67 of the liner side wall, and may be formed, forexample, from beryllium, titanium, or the like. Mounting the window 22to the liner 64 rather than to the framework 62 increases the conductionof heat away from the window, where overheating is often prone to occur,due to the deflection of electrons from the target area of the anode.For example, a shelf (not shown) is milled into an outer surface 73 ofthe liner side wall 66. The window 22 is then brazed, welded, orotherwise attached to the shelf.

Alternatively, the window 22 is mounted to the framework 62, withclosely adjacent thermal passages 60 of copper to aid in heat removal.In this case the framework is hermetically sealed around the window tothe liner, with a hole in the liner for the x-rays to pass through.

The framework 62 of the vessel is similarly shaped to the liner 64 andincludes a side wall 74 with a cylindrical wall portion 75 and anannular base portion 76 from which the flange 57 depends. The baseportion 76 defines a central aperture 78 concentric with the opening 70in the liner and of similar size. The liner aperture 70 and frameworkaperture 78 together define the central aperture 56 of the vessel.

Slots 80, 82 are formed in the wall portion 75 and base portion 76,respectively, which serve as thermal windows to the liner 64 containedwithin the framework. The slots 80, 82 (twelve angularly spaced slots ofeach type are illustrated in FIG. 3) are sized to optimize thermaltransfer from the vessel 40 while allowing the liner 64 to besubstantially thinner than a comparable copper frame formed without aframework. While the illustrated slots 80, 82 are generally ovoid, othershapes and sizes of slots are contemplated. The thermally conductivepathways 60 are defined by portions of the underlying liner 64 which areexposed to the cooling liquid through the slots 80, 82. As illustratedin FIG. 4, at least one of the slots 80A is positioned over the window22 so that x-rays leaving the frame 16 pass through the slot withoutinterference by the framework.

With continued reference to FIG. 3, the framework 62 includes aplurality of ribs 84, intermediate each of the slots 80, which extendparallel with the axis of rotation X of the anode. The ribs 84 areconnected, at upper and lower ends, to annular, ring-like portions 86,88 of the framework. In the base portion 76, radially extending ribs 90,intermediate the slots 82, join the annular frame portion 86 with aninner annular frame portion 92, adjacent the aperture 78.

It will be appreciated that other configurations of a constrainingframework are contemplated. In its simplest form, the framework servesas a cage and comprises an upper annular portion 86 and an inner annularframe portion 92, connected by ribs. Preferably there are a minimum ofthree ribs 84, 90, which are angularly spaced around the vessel 40. Ribs90 may simply be extensions of ribs 84.

To improve heat flow from the liner 64, the exterior surface 73 of theliner, e.g., in the regions of the slots 80, 82, is provided with fins,projections, or other surface features 94 which increase the surfacearea of the liner that is exposed to the cooling oil. FIG. 4 illustratesa surface 73 with fins 94, by way of example. Although some heat flowsto the cooling fluid through contact with an outer surface 95 of theframework, the bulk of the heat transfer from the vessel 40 occursthrough the thermal passages 60 formed at the slots 80, 82.

The framework 62 is preferably attached to the liner 64, at least atselected points. In the embodiment of FIGS. 2 and 3, an inner surface 96of the framework 62 is attached to the outer surface 73 of the liner.This attachment helps to minimize relative movement between the linerand the framework during heating and cooling of the x-ray tube 11 andunder the forces generated by rotation of the x-ray tube about thepatient. In one embodiment, the framework is brazed to the liner, eitherover the entire area of contact, or at select locations. For example,the framework 62 is optionally brazed to the liner to form hermeticseals at sealing regions 97, 98 adjacent the annular portions 86, 92(FIG. 2). Other methods of attachment are also contemplated. Forexample, diffusion bonding or explosion bonding is used to bond theframework to the liner. In diffusion bonding, a high pressure is used tosqueeze the two components together, preferably accompanied by a hightemperature. In explosion bonding, an explosive charge is used to forcethe liner and framework into contact.

In another method of attachment, the framework 62 is formed first andthe liner 64 is subsequently cast onto the framework (or vice versa).Optionally, the high thermal conductivity liner can encompass thestructural framework. The cast liner can then be machined, asappropriate, without the need for an interior support structure toprevent deformation of the liner. In yet another method, suitably sizedsheets of material for the liner and framework are prepared (optionallywith the slots 80, 82 and apertures 70, 78 cut out). The two or morelayers are pressed with a ram into a mold, forming the shape of thevessel under high pressure.

As shown in FIG. 2, the side wall 74 of the framework 62 extendsslightly above the side wall 66 of the liner 64 to provide a weld flange100 by which the vessel 40 is welded or otherwise rigidly attached tothe plate 44.

In the embodiment of FIGS. 2-4, the framework 62 is entirely outside theliner 64 and thus is not generally exposed to the vacuum environment.Accordingly, the framework material, such as stainless steel, need notbe free of impurities of the type which tend to outgas in the vacuumenvironment. However, where portions of the framework are exposed to thevacuum environment, the framework material is preferably selected tominimize impurities which tend to outgas. Stainless steels, Inconel™,nickel alloys, titanium, and Kovar™ are suitable vacuum compatiblematerials. Positioning the liner 64 in contact with the vacuumenvironment provides an inner surface 102 which absorbs heat relativelyuniformly.

With reference now to FIGS. 5 and 6, where similar elements are numberedwith a primed suffix (′), a vessel 40′ includes an outer liner 64′formed of a conductive material, and a framework 62′, formed of astructural material. The framework and liner are similar to liner 64 andframework 62 of FIGS. 2-3, except in that the framework 62′ is locatedinterior to the liner 64′ , with an outer surface 95′ of the frameworkattached to an inner surface 102′ of the liner. The entire outer surface73′ of the liner, in this embodiment, is in direct contact with thecoolant. Other features of the vessel 40′ can be otherwise similar tothe embodiment of FIGS. 2-3. Since the stainless steel framework 62′ isexposed to the vacuum environment, the framework material is preferablyfree or substantially free of impurities which have a tendency to outgasin the vacuum environment. Portions of the liner 64′ are also directlyexposed to the vacuum environment and these too are preferably free ofoutgassing impurities.

The combination of copper and stainless steel is particularly suitablefor forming the liner 64, 64′ and framework 62, 62′, respectively. Theyhave relatively similar thermal expansion coefficients. The coefficientfor copper is about 20×10⁻⁶ cm/cm/° C., which is slightly higher (about10% higher) than that of stainless steel. Where the copper liner 64 isinterior to the steel framework 62, this difference in thermal expansionhas little or no effect on the structural stability of the vessel, sincethe steel acts to prevent or substantially limit any expansion of thecopper liner which exceeds that of the stainless steel. Even where theliner 64′ is placed exterior to the framework 62′, the welding or otherform of attachment of the liner to the framework helps to offset anytendency of the copper to expand away from the steel.

Similarly, although copper begins to exhibit noticeable material creepat a load of about 70-210 Kg/cm², the comparable value for stainlesssteel is at least about 700 Kg/cm². The stainless steel framework thusprovides a vessel 40, 40′ which is resistant to creep. Stainless steelalso has a resistance to bending which is 30-40% higher than that ofcopper. As a result, the vessel has, in large part, the structuralstrength and rigidity of a steel vessel, while retaining, in large part,the thermal conductivity of a copper vessel.

In another embodiment (not shown), rather than having slots 80, 82, 80′,82′ through which the thermal passageways in the liner make directcontact with the cooling liquid, thinned regions of the framework areprovided of a similar shape and size to the slots, which serve asthermal windows. The thinned regions have a wall thickness which is lessthan half that of the ribs, preferably less than 30%. The thinnedregions are thin enough that they do not appreciably limit the heat flowtherethrough, but thick enough to provide a gas impermeable barrier.

In yet another embodiment (not illustrated), a framework similar toframework 62, 62′ is sandwiched between respective inner and outerliners similar to liners 64 and 64′.

With reference now to FIGS. 7 and 8, where similar elements are numberedwith a double primed suffix (″), a vessel 40″ includes an inner liner64″ formed of a conductive material, and a framework 62″ formed of astructural material. The framework and liner are similar to liner 64 andframework 62 of FIGS. 2-3, except as noted. In this embodiment, theframework 62″ is formed of round or tubular wire. Ribs 84″ in the formof spokes (three in the illustrated embodiment), are defined by piecesof the wire, which are brazed, welded or otherwise attached at endsthereof to annular portions or support rings 86″ 92″. It is appreciatedthat the ribs need not be round, and many other shapes are possible. Thesupport rings, in turn, are brazed or welded to the liner 64″. The uppersupport ring 86″ is also brazed, welded or otherwise attached to thecathode plate 44. The lower support ring 92″ defines a flange 57″ whichis attached to the lower portion 58 of the frame housing the bearing(FIG. 1). Spaces 80″ between the spokes and support rings 86″, 92″define thermal windows through which the cooling oil makes thermalcontact with the chamber, via the thermally conductive material.Optionally, additional subframework elements which are significantlymore deformation resistant than the liner, but significantly morethermally conductive than the framework, can be used to supplement theframework.

With reference now to FIGS. 9 and 10, where similar elements arenumbered with a triple primed suffix (′″) and new elements are given newnumerals, a vessel 40′″ includes an inner liner 64′″ formed of aconductive material, and a framework 62′″, formed of a structuralmaterial. The framework and liner are similar to liner 64 and framework62 of FIGS. 2-3, except as noted. The framework 62′″ is spaced from theliner 64′″, except at regions of attachment 97′″, 98′″, to provide anannular cooling passage 120 for cooling oil to pass between theframework and the liner. The oil may be directed through the coolingpassages by walls (not shown) constructed between the liner andframework to optimize the cooling efficiency of the oil. The conductiveliner may have projections at the points of attachment to maintain theoil gap width. Cooling fluid inlet and outlet ports 122, 124 are formedin the framework 62′″ through which cooling fluid from the x-ray tubehousing is directed through the cooling passage. Optionally, the coolingfluid inlet port 122 is connected with a pump (not shown) which suppliespressurized cooling fluid to the passage 120.

In this embodiment, thermal windows are defined by the outlet ports 124,for thermal contact between the cooling oil and the chamber 14, via theliner. The entire volume of the liner can be considered as a thermalpassage 60′″. While there are no slots analogous to slots 80, 82 in theembodiment illustrated, it is also contemplated that slots similar toslots 80, which are preferably spaced from the inlet port 122, may beprovided in addition to, or in place of the outlet ports 124.

The invention has been described with reference to the preferredembodiment. Modifications and alterations will occur to others upon areading and understanding of the preceding detailed description. It isintended that the invention be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

1. An x-ray tube (11) comprising: a frame (16) which encloses anevacuated chamber (14); an anode (12) disposed within the evacuatedchamber; the frame including a vessel (40, 40′, 40″, 40′″) whichsurrounds the anode, the vessel being defined by a combination of amaterial with high thermal conductivity and lower deformation resistanceand a material with high deformation resistance and lower thermalconductivity.
 2. The x-ray tube according to claim 1, wherein the vesselincludes: a liner (64, 64′, 64″, 64′″) formed from a thermallyconductive material which at least partially defines the evacuatedchamber; and a framework (62, 62′, 62″, 62′″) which supports the linerand is formed from a structural material, the framework defining atleast one thermal window (80, 80′, 80″, 82, 82′, 124) therein throughwhich the liner is in thermal contact with both the evacuated chamberand a surrounding cooling fluid.
 3. The x-ray tube according to claim 2,wherein the framework and the liner are concentric.
 4. The x-ray tubeaccording to claim 2, wherein the framework (62, 62″, 62′″) surroundsthe liner (64, 64″, 64′″).
 5. The x-ray tube according to claim 2,wherein the thermal window comprises at least one slot (80, 80′, 80″,82, 82′) defined in the liner (64, 64′).
 6. The x-ray tube according toclaim 5, wherein the at least one slot includes a plurality of angularlyspaced slots (80, 80′, 80″, 82, 82′).
 7. The x-ray tube according toclaim 2, wherein the thermally conductive material has a thermalconductivity which is at least twice that of the structural material. 8.The x-ray tube according to claim 2, wherein the structural material hasa yield strength which is at least twice that of the thermallyconductive material.
 9. The x-ray tube according to claim 2, wherein thestructural material includes stainless steel.
 10. The x-ray tubeaccording to claim 2, wherein the thermally conductive material includescopper.
 11. The x-ray tube according to claim 2, wherein the linerincludes a cylindrical side (67, 67′, 67′″), and a base (68, 68′, 68′′)and wherein the framework includes a cylindrical side (75, 75′, 75′″)and a base (76, 76′, 76′″), the side of the liner being joined to theside of the framework.
 12. The x-ray tube according to claim 2, whereinone of the liner and the framework is received within the other of theliner and the framework.
 13. The x-ray tube according to claim 2,wherein the liner defines a central aperture (70, 70′, 70″, 70′″) andthe framework defines a central aperture (78, 78′, 78″, 78′″), the anodeincluding a shaft (17) which extends through the central apertures. 14.The x-ray tube according to claim 2, wherein the liner and the frameworkdefine a fluid flowpath (120) there between for the cooling fluid tocontact the liner.
 15. The x-ray tube according to claim 2, furtherincluding a plate (44) which closes an end (42) of the vessel (40, 40′,40″, 40′″), the plate defining an aperture (46) through which a cathodeassembly extends for emitting electrons that pass between a cathode andthe anode.
 16. The x-ray tube according to claim 2, wherein the vesselcomprises a laminate of the conductive and structural materials.
 17. Anx-ray tube assembly (10) comprising: the x-ray tube(11) of claim 1; anda housing (30) surrounding at least a portion of the x-ray tube, thehousing containing the cooling fluid.
 18. A method of transferring heatfrom an x-ray tube (11) to a surrounding cooling fluid comprising;conducting heat from an evacuated chamber (14) through a liner (64, 64′,64″, 64′″) of the x-ray tube formed from a thermally conductivematerial; restraining the liner against deformation with a structuralframework (62, 62′, 62″, 62′″).
 19. The method according to claim 18,wherein the structural framework defines at least one thermal window(80, 80′, 80″, 82, 82′, 124), the heat flowing directly between theliner and the surrounding cooling fluid in the thermal window.
 20. Anx-ray tube (11) comprising: a thermally conductive liner (64, 64′, 64″,64′″) which spaces an evacuated chamber (14) of the x-ray tube from asurrounding cooling fluid; a structural framework (62, 62′, 62″, 62′″)forming a cage which reinforces the liner against deformation.
 21. Thex-ray tube of claim 18 further including an anode (12) mounted in theevacuated chamber.